The present invention generally relates to the continuously variable transmission art. More particularly, the present invention relates to temperature compensation in continuously variable transmissions and to providing temperature compensation to maintain desired system response characteristics for regulating fluid control, clutch control and belt ratio control.
The art contains numerous examples of the operation and construction of continuously variable transmissions (CVT's), e.g., U.S. Pat. No. 4,522,086 and U.S. Pat. No. 4,458,318, entitled Control System for Continuously Variable Transmission, and Control Arrangement for a Variable Pulley Transmission, respectively. These patents, generally describe the mechanics and controls for a CVT system utilizing two adjustable pulleys, each pulley having at least one sheave which is axially fixed and another sheave which is axially movable relative to the first sheave. A flexible belt of metal or elastomeric material intercouples the pulleys. The interior faces of the pulley sheaves are beveled or chamfered. Thus, as the axially displaceable sheave moves relative to the fixed sheave, the distance between the sheaves and, thus, the effective pulley diameter may be adjusted. The displaceable sheave includes a fluid constraining chamber for receiving fluid to move the sheave and thus change the effective pulley diameter; as fluid is exhausted from the chamber, the pulley diameter changes in the opposite sense. Generally, the effective diameter of one pulley is adjusted in one direction as the effective diameter of the second pulley is varied in the opposite direction, thereby effecting a change in the drive ratio between the input shaft coupled to an input pulley and an output shaft coupled to an output pulley. The ratio changes continuously as the pulley diameters vary. Such transmissions frequently are referred to in the art as a continuously variable transmission, CVT.
Through the years various developments have refined and improved the hydraulic control system which is used to pass fluid into the fluid holding chamber of each adjustable pulley. An example of such a hydraulic system is shown in U.S. Pat. 3,115,049--Moars. In that patent, control of the secondary pulley adjustable sheave regulates the belt tension, while a different circuit regulates fluid into and out of the primary sheave to regulate the transmission ratio. U.S. Pat. 4,152,947 van Deursen et al.--also describes control of a CVT. In both systems, the line pressure of the fluid applied to hold the belt tension by pressurizing the secondary chamber is kept at a relatively high value. An improved control system subsequently was developed to reduce the main line fluid pressures supplied to the secondary sheave chamber as a function of torque demand. This improved system is described and claimed in an application entitled "Control System for Continuously Variable Transmission," now U.S. Pat. 4,522,086, assigned to the assignee of this application. Further work resulted in an improved control system which reduced the line pressure applied to the secondary chamber to a lower, safer operating pressure and also provided a lower control pressure for other portions of a hydraulic control system. This system is described in an application entitled "Hydraulic Control System for a Continuously Variable Transmission", Ser. No. 421,198, filed Sept. 22, 1982, which issued Dec. 15, 1987, as U.S. Pat. No. 4,712,453 and assigned to the assignee of this application.
Additional significant advances in CVT control systems have been described and claimed in another application also entitled "Hydraulic Control System for a Continuously Variable Transmission," Ser. No. 717,913, filed Mar. 29, 1985, which issued Jan. 12, 1988 as U.S. Pat. No. 4,718,308, and assigned to the assignee of this application.
Another U.S. Pat. No. 4,648,496, entitled "Clutch Control System for a Continuously Variable Transmission" issued Mar. 10, 1987, further describes control logic techniques for regulating pressure at a clutch in a CVT system to provide the desired torque transfer from an engine to a vehicle drive line. In that system, clutch control focuses on logical recognition of one of a number of operating modes.
A related, co-pending application entitled "Continuously Variable Transmission Clutch Control System", Ser. No. 25,391, filed Mar. 13, 1987, assigned to the assignee of present application discloses an improved clutch controller for use in CVT applications. A second, related, co-pending application entitled "Special Start Technique For Continuously Variable Transmission Clutch Control", Ser. No. 25,476, filed Mar. 13, 1987, assigned to the assignee of the present application, discloses a unique technique for regulating the clutch pressure control signal in a CVT system during selected driving conditions, e.g. skidding on ice.
Another, related, copending application entitled "Ratio Control Technique For Continuously Variable Transmission Systems," Ser. No. 25,389, filed Mar. 13, 1987, assigned to the assignee of the present application, discloses a unique, technique for controlling the transmission belt ratio in a continuously variable transmission system.
The teachings of each of the above referenced patents and applications are incorporated herein by reference in terms of background to the present invention, although none of the previously envisioned systems addresses implementation of the compensation techniques disclosed herein.
CVT systems typically rely upon hydraulic fluid to implement various control function, such as belt ratio control, line pressure control and clutch control. The viscosity and hence the responsiveness of the hydraulic fluid to flow, however, vary substantially over the range of system operating temperatures which may be expected for a typical application of a continuously variable transmission. Variation in the viscosity of the hydraulic fluid results in an undesirable variation in the system response which creates a negative impact on the system operation, reliability and, ultimately, stability.